Process of eliminating iron and other impurities from caustic soda



Patented Mar. 29, 1938 2 112 13 UNITED STATES PATENT OFFICE PROCESS OFELIMINATING IRON AND OTHER IMPURITIES FROM CAUSTIC SODA Domingo Lopez,Charleston, W. Va., assignor to Westvaco Chlorine Products Corporation,New York, N. Y., a corporation of Delaware No Drawing. Application March8, 1934, Serial N0..714,737

3 Claims. (Cl. 23184) This invention relates to a process by means Inthis formula, 69 is the percent NaOH in the of which the iron present inthe caustic soda pure monohydrate crystals and- (m) the percent liquorsproduced by electrolytic or any other of NaOH in the residue motherliquor. It will be means, as soluble impurity, is reduced to the pointclearly seen that y the percent of mother liquor of not beingobjectionable to the most exacting frozen on the crystals increases when(m) in- 5 requirements, if so desired. creases up to the limit 69, andthat naturally The demand in the market for caustic soda occurs when theconcentration of the liquor to containing a very minimum of iron hasstimulated crystallize runs high. (The concentration m, the producer tofind new ways for the elimination of the mother liquor runs about 1.5 to2% below of that and other impurities. the concentration of theconcentrate or liquor to 10 It is well known that most of the salts canbe crystallize.) refined by crystallization followed by a mechan- Thisrelation may be deduced as follows: ical separation of the crystals fromthe mother Let:

liquor.

Liquid caustic can be obtained by evaporation gf ig zs gg fi $9 g 15 todifierent concentrations, each one capable of g z g s producing crystalswhen the solution is cooled to is NaOH i proper l these Crystals Wrec=overall percent by weight of actual aggressolved produce a liquid ofgreater purity than gate crystals produced which is NaOH the originalone, nevertheless from the commeryzpercent of total weight of aggregatecrys 20 cial and from the technical point of view not all theconcentrations are suitable. I have discovffi i f Whlch 1S frozen motherered that by concentratmg a solution of caustic wztot a1 W eight ofaggregate crystals produced. soda contaimng about 50% NaOH and about1.2% D t0ta1 weight of frozen mother H r of NaCl, up to 57%concentration as minimum T quo on and 65% concentration as maximum, andthen crystals produced causing crystallization by extraction of heat,the Then crystals as actually produced when separated CW=K(WD) from themother liquor are practically iron free, whence! but when concentratingabove 65% or even at 30 65% concentration, the impurities are separatedfrom the crystals to a much smaller extent than and Since forconcentrations below 65%. This occurrence 2 is natural and isanticipated by the theory; the W actual crystals that are collected asthe product 35 of the crystallizing process are of the kind of Y puremonohydrate crystals, (NaOI-LHzO) together with a certain amount ofmother liquor The quantity K, which is the percent by weight frozen ontheir surface. Now these crystals so of pure monohydrate crystalsNaOI-LHzO which produced with frozen mother liquor on their suris NaOH,is equal to 40 face contain a nearly constant overall percent of NaOH asthey are delivered, as from a centrif- 987 or 1 ugal crystal separatingmachine in most cases 40+18 running from 67.5 percent to 68 plus NaOH,

and for a determined type of centrifugal ma- If We take the Value of cas we have 45 chine and. for set conditions this percent of NaOH 7 5 5content can be considered as fixed. If we adopt T 67.5%, NaOH as theoverall value of NaOH content of the crystals and frozen mother liquorThis 15 the equatwn of an equllateml hyperbola so produced in the actualcrystals, the amount and it is found that the Value of 3/ increases 5 ofmother liquor attached to the crystals will be rapidly when? gets aslarge as A Value expressed a f n w c as high as 67.5 can be obtained byusing a good centrifugal crystal separating machine and care- 6967.5 1.5iul handling.

9- 69-m This equation indicates the degree of purifica- 55 tion that maybe expected in using my process. It is obvious that concentrations ofNaOH in the concentrate closely approaching as high a concentration as69% should be avoided, since they will correspond to large values of 11,and, therefore, large amounts of mother liquor with its includedimpurities frozen to the crystals.

The viscosity increases with the caustic concentrations. The greater theviscosity, the harder to eliminate the mother liquor and the iron whichit contains.

In a process in which the actual aggregate crystals produced contain67.5% NaOH, the NaOI-I content of the mother liquor becomes equal tothat of the concentrate, that is, for a concentrate of 67.5% NaOH. Inother words, in such a case, concentrate, mother liquor and crystalswould show the same analysis of 67.5% NaOH.

In a like process in which the concentrate used contains only 60% NaOH,the mother liquor is of 56.79% NaOH.

For the reasons already stated, I prefer to use a concentrate ofconcentration from 62% to 63% NaOH, which permits to obtain crops ofcrystals as high as 40% of the weight of the 63% caustic solution usedwithout introducing any difficulty in the manipulation and delivering aproduct which when dissolved and diluted to 50% would contain no morethan 10 to 12 parts per million of iron as Fe, if the liquor of 50% NaOHand 1.2 percent NaCl did not contain more than 40 to 50 parts Fepermillion at the start.

I found also that the degree of purification can be increased by washingwith a pulp of caustic made out of crystals and liquor not containingmore than to parts of iron per million, and no more than 1.2% NaCl, ormoistening the chamber of the centrifugal crystal separating machinewith a mist of water or steam. The other impurities susceptible of beingremoved by identical process decrease in an almost parallel manner,although at high concentrations when the consistency of the pulp hasbeen allowed to go too far, the salt, for instance, is not eliminated asefficiently as is the iron.

The process may be illustrated as follows:

Caustic soda liquor containing about NaOH, not more than 1.2% NaCl andabout 40 parts per million of iron as Fe, usually starting with coldliquor to keep down the NaCl content,

is concentrated byheating in a non-iron metal falls to 48? C. (for 57%concentration) and to 60 C. (for 63% concentration) respectively, themass being stirred in order to help dissipation of heat, and, when thecrystallization has started, to prevent the formation of clusters. Thetemperature is allowed to drop down to 43 C. in the first case and to 57C. in the second to secure. a yield of about 25% in the first and ofabout 35% in the second case. During the cooling and crystallization themass will in practice be flowing from one end to the other of thecrystallizer and dropping into a crystal separating centrifugal machine.At those temperatures the pulp is delivered into a centrifugal crystalseparating machine warmed and kept at the temperature of the crystals orslightly thereabove. The mother liquor is brought back into the systemor it is brought to a second crystallization if the concentration ofimpurities is not too great.

It is obvious that my invention may be varied somewhat as to detailswithout departing from the spirit thereof.

Having thus described the invention, what is claimed is:

1. In purifying caustic soda by crystallization of NaOI-II-IzO fromcaustic solutions containing dissolved'iron, a process of producing acaustic low in iron which comprises evaporating the solution to aconcentration of substantially 63 per cent NaOH, cooling theconcentrated solution to substantially 60 C., crystallizing out about 35per cent of the caustic as monohydrate and separating from the crystalsa mother liquor containing about per cent or more of the dissolved ironof the original solution.

2. A process of producing a purified caustic soda from an evaporatedelectrolytic cell liquor containing approximately 50 per cent NaOH, 1.2per cent NaCl and 40 parts dissolved iron per million which comprisesconcentrating the liquor until it contains substantially 63 per centNaOH, cooling the concentrated liquor to substantially 60 C.,crystallizing Na'OHHzO from the cooled liquor and centrifuging from thecrystals a mother liquor containing most of the dissolved iron.

3. In the purification of caustic soda, a method of removing iron andsalt which comprises preparing a solution containing approximately 63per cent NaOH, cooling this solution to approximately 60 C. tocrystallize the caustic as monohydrate and separating the crystals froma mother liquor carrying iron and salt.

DOMINGO LOPEZ.

